Measurement of intracrystalline diffusion of nitrogen in zeolites NaX and NaCaA using pulsed field gradient n.m.r.

ELSEVIER

Measurement of intracrystalline diffusion of nitrogen in zeolites NaX and NaCaA using pulsed field gradient n.m.r. N.-K. B%r,* P.L. McDaniel,+ C.G. Coe,+ G. Seiffert,+* and J. tiger* *Fakulttit fir Physik und Geowissenschaften der Universitiit Leipzig, Leipzig, Products and Chemicals, Inc., Allentown, Pennsylvania USA

Germany ‘Air

The measurement of intracrystalline diffusion (Di,,,) of nitrogen in zeolites by 15N pulsed field gradient (p.f.g.) n.m.r. is complicated by the small magnetogyric ratio. This causes a large reduction in signal sensitivity, necessitating a significant enhancement of the field gradient pulse intensity in comparison with ‘H p.f.g. n.m.r. Owing to recent progress in measuring techniques, these technical problems were overcome and direct measurement of the intracrystalline selfdiffusion coefficients of nitrogen in l2- to 15-pm crystals of zeolite NaX and NaCaA in a temperature range between 135 and 300 K was completed. The intracrystalline diffusion for both these zeolites was found to be a hundred times slower than the long-range diffusion within the crystal bed. At present, these measurements are not possible for typical A- and X-type zeolites, which have diameters less than 10 pm. 0 Elsevier Science Inc. 1997 Keywords:

lntracrystalline

diffusion;

NaX;

NaCaA;

nitrogen;

pulsed

INTRODUCTION Pulsed field gradient (p.f.g.) n.m.r.‘.” has proven to be an effective tool for studying intracrystalline zeolitic diffusion2-4 by directly monitoring molecular displacements within adsorbate-adsorbent systems. The most favorable conditions for application of this method are provided by ‘H n.m.r. Initially, p.f.g. n.m.r. studies on zeolites were limited to hydrogen-containing species.” In the last few years, however, p.f.g. n.m.r. has also been performed using lgF n.m.r.6 whose sensitivity is similar to ‘H n.m.r., as well as 13C and ‘2gXe n.m.r.’ Though 15N n.m.r. has been successfully applied to the study of the conformation of nitrogen-containing species in zeolites,” only recently have 15N p.f.g. n.m.r. studies of nitrogen diffusion in commercial zeolites of type NaX and NaCaA been completed.” These studies were devoted to “long-range” diffusion, i.e., to the rate of molecular propagation through a bed of zeolite crystallites. The obtained diffusivities are in satisfactory agreement with the prediction based on simple gas-kinetic theory approaches for the mass transfer through the intercrystalline space. Further, because the minimum displacements covered in these studies were approximately 20 pm, no information about intracrystalline diffusion could be obtained. p.f.g. n.m.r. diffusion studies using ‘“N n.m.r. are complicated by the small magnetogyric ratio (y [‘“N] =

field

gradient

n.m.r.

2.7106 . 10’ T-‘s-l), which is one order of magnitude smaller than that for hydrogen (y[‘H] = 2.6752 . 10’ T-‘s-l). This results in a reduction in the signal sensitivity by a factor of 1.04 * 10W3 in comparison with hydrogen. Moreover, one needs to consider that the natural abundance of 15N is only 0.37% in comparison with 99.98% for ‘H. This leads to a further dramatic reduction in signal intensity and requires that 15N p.f.g. n.m.r. measurements be made using 15N-enriched compounds. The measurement of molecular diffusion using p.f.g. n.m.r. experiments is based on observation of n.m.r. signal attenuation (*) after an appropriate sequence of rf pulses and inhomogeneous magnetic field (field gradient) pulses are applied. Spin-echo attenuation in a homogeneous system obeys the relation1’2’5 q = exp[-y2

where 6, g, and A separation of the spectively. D is the der study. Equation stein relation

reprint

requests

Geowissenschaften, D-04103 Received

Leipzig, 8 March

to Prof.

University Germany. 1996;

accepted

Zeolites 18:71-74, 1997 0 Elsevier Science Inc. 1997 655 Avenue of the Americas,

New

Ktirger

at Fakulttit

of Leipzig, 27 August

York,

NY

fiir

Physik/

Linkstrasse 1996

10010

5,

(1)

denote the width, amplitude, and magnetic field gradient pulses, reself-diffusivity of the molecules un(1) may be modified using the Ein-

(r2(t))=6~t

(2)

relating the mean square vation time, yielding 9 = exp[-y2

Address

6’ g’ D(A - s/3)]

displacement

and the obser-

6’ $ (r2 (A - 6/3))/6]

Any observation of molecular n.m.r. implies that the exponent of Equation (3) must be equal

(3)

displacements by p.f.g. on the right-hand side to or greater than one.

0144-2449/97/$17.00 PII SO144-2449(96)00114-5

lntracrptalline

diffusion

of nitrogen:

N.-K. Bdr et a/.

Consequently, the sensitivity of 15N p.f.g. n.m.r. with respect to small molecular displacements, is a factor of 10 smaller than in ‘H p.f.g. n.m.r. Further, according to Equation (2), this results in increasing the lower limit of the diffusivity by two orders of magnitude compared to ‘H p.f.g. measurements. An unambiguous observation of intracrystalline diffusion (Dint,,) by p.f.g. n.m.r. is only possible if the mean molecular displacement covered during the observation time (A) of the p.f.g. n.m.r. experiments is much smaller than the mean crystallite radius. Otherwise the observed displacements would be significantly influenced by the crystallite boundaries. This fact, together with the requirement that the exponent on the right-hand side of Equation (3) must be greater than one, imposes a lower limit on the size of the zeolite crystallites. These two factors require the minimum crystallite diameters for studying intracrystalline diffusion by “N p.f.g. n.m.r. to be a factor of ten larger than in rH p.f.g. n.m.r.. Typical minimum crystallite sizes for ‘H p.f.g. n.m.r. measurement of intracrystalline diffusion are of the order of 1 pm.’ Therefore, even with the most powerful p.f.g. n.m.r. spectrometers available, 15N p.f.g. n.m.r. measurement of intracrystalline diffusion in zeolites requires zeolite crystallites with a mean diameter of at least 10 pm. This precludes the use of commercially available X- and A-type zeolites. Using the home-built p.f.g. n.m.r. spectrometer FEGKIS 4001’ we carried out the first p.f.g. n.m.r. measurements of intracrystalline self-diffusion of nitrogen in zeolites. The zeolites chosen for this study are known as 13X (NaX) and 5A (NaCaA) and are used commercially to produce oxygen by pressure swing adsorption. In addition, these adsorbents are examples of smallpore (NaCaA) and large-pore (NaX) zeolites.

EXPERIMENTAL, We used powders of zeolite NaCaA (71% Na exchanged by Ca) with a diameter of 15 microns and NaX with a diameter of 12 microns, kindly provided by S.P. Zhdanov (St. Petersburg).” The samples for the p.f.g. n.m.r. experiments were prepared by heating 12-mmhigh layers of zeolites in 8-mm diameter glass tubes. The temperature was increased at a rate of 10 K h-’ under vacuum. After maintaining the samples at 673 K and at a pressure lower than 0.01 Pa for 24 h, the samples were loaded with a known amount of 15Nenriched nitrogen gas (99 + %‘“N,, Isotec) and sealed. The amounts adsorbed corresponded to loadings of 0.5 and 1.6 molecules per cage for NaCaA and 0.4 and 1.3 molecules per cage for NaX. These loadings correspond to adsorbate pressures of 0.5 and 2.5 atm at room temperature.’ The p.f.g. n.m.r. measurements were performed on the home-built p.f.g. n.m.r. spectrometer FEGIUS 400 at a proton resonance frequency of 400 MHz with a maximum field gradient amplitude of 24 T/m.” To ensure mechanical stability, the zeolite powder was compacted under a constriction in the sample tube.‘* In general, we applied the Hahn echo with an observation time A = 2 ms. In one systematic study, we varied

72

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1997

the observation time between 1 and 10 ms, and for A 2 2 ms, applied the stimulated echo.‘,’

RESULTS AND DISCUSSION At temperatures below about 190 K, the spin-echo attenuation of the adsorbed nitrogen molecules is in complete agreement with the behavior predicted by Equation (1). We can assume, therefore, that the mean diffusion paths covered during A are much less than the crystallite radius, so that the majority of the N, molecules do not encounter the crystallite boundary and behave as if in an infinite homogeneous medium. This assumption can be confirmed quantitatively by using Equation (2). For nitrogen in NaX at a loading of 1.3 molecules, e.g., at T = 163 K, the root mean square displacement during an observation time of 3 ms is found to be equal to 2.85 pm, which is in fact much smaller than the crystallite diameters. With increasing temperature, however, more and more molecules are able to leave the individual crystallites during the observation time. This leads to a first steeper decay in the echo attenuation whose slope is a measure of the long-range diffusivity (D,,). We analyzed this slope in all cases where its contribution to the overall echo attenuation was larger than I5%. As an example, Figure 1 presents the spin-echo attenuation of nitrogen in zeolite NaX for a sorbate concentration of 1.3 molecules per cage at 208 K. From the slope of the first steep decay one may deduce the lon range diffusivity.2,5 The resulting diffusivity of 1 * lo- g; m*/s corresponds to a root mean square (rms) displacement of 34 pm, which is about three times larger than the crystallite diameter. The slope of the second decay in the echo attenuation is determined by the diffusion inside the zeolite crystallites. The resulting intracrystalline diffusivity (Dint7J of 3 * lo-’ m*/s corresponds to an rms displacement of 6 pm. This value is of the order of the mean radius of the crystallites and

‘i w

1 . . .

0.1 I

: 0

1

50

I 100

I

I

150

200

( 6 g ) 2 / 10-S T 2 S2 m-2 Figure 1 Echo attenuation plot ment of nitrogen in zeolite NaX at molecules per cavity at 208 K for The measurements were carried gradient width 6.

of 15N p.f.g. n.m.r. measurea sorbate concentration of 1.3 an observation time of 2 ms. out with different values of

lntracrystalline

confirms lites.

that the displacements

are within

(R2) D Test7-__ - JjA

(4)

Inserting the relevant data for Da,, (1 * lo-’ m”/s) and A = 2 ms into Equation (4) gives a value of (@)‘/s = 3.5 pm, which is smaller than the crystallite radius, meaning that the nitrogen molecules are not restricted by the crystallite boundary. Moreover, measurements with observation times of 1 and 10 ms yield the same diffusivity as those at 2 ms. This is in striking contrast to Equation (4)) where the effective diffusivity is proportional to the reciprocal value of the observation time. It is rather unlikely, therefore, that the constancy of D with further increasing temperature is caused by molecular confinement by crystallites. As an alternative, the deviation from an Arrhenius dependence of D,,, with increasing temperature may be due to a decrease in the amount adsorbed within the sample. Because there is a finite amount of nitrogen within the n.m.r. sample tube, with increasing temperature the fraction of molecules in the gas phase increases lE-6

1

1 3

I 4

5

6

7

1 6

IOOOK I T Figure 2 Temperature dependence of rystalline self-diffusion (solid symbols) sion (open symbols) of nitrogen in NaX triangles are for N, loadings of 0.4 and respectively.

of nitrogen:

N.-K. Blr et al.

the crystal-

Figures 2 and 3 show the obtained diffusivities as a function of temperature. The coefficients of long-range diffusion (open symbols, determined from the first steep slope) and intracrystalline diffusion (solid symbols, taken from the second slope) are included. For sufficiently low temperatures, the observed displacements are exclusively due to intracrystalline diffusion. At temperatures above 190 K, long-range diffusion is also observed. It is remarkable that with further increasing temperature the intracrystalline diffusivities tend to approach a limiting value. In previous studies such a temperature dependence could often be attributed to molecular restriction within the zeolite crystallites. In the case of molecular restriction the coefficient of intracrystalline diffusion is actually a restricted one (Drest7), being related to the mean square radius of the confining region by the equation2,5

IE-11

diffusion

the coefficients of intracand of long-range diffuzeolite. The squares and 1.3 molecules per cavity,

iE-7:

IE-10

lE-11 iI 3

4

5

6

7

8

1OOOKlT

Figure 3 Temperature dependence of the coefficients of intracrystalline self-diffusion (solid symbols) and of long-range diffusion (open symbols) of nitrogen in NaCaA zeolite. The squares and triangles are for N, loadings of 0.5 and 1.6 molecules per cavity, respectively.

and becomes significant close to room temperature. Because the diffusivities tend to decrease with decreased total concentration, a decrease in the number of sorbed molecules with increasing temperature may, in fact, lead to a deviation of the diffusivities from the typical Arrhenius dependence. The error bars shown in Figures 2 and 3 reflect the experimental uncertainties of 15N p.f.g. n.m.r. measurements of intracrystalline diffusion. Within these limits, the absolute values of the diffusivities coincide and the activation energies of diffusion (10.8 k 1.5 kJ/ mol in NaX/8.5 + 1.5 kJ/mol in NaCa4) agree within the limits of accuracy. This situation is analogous to the adsorption properties, where the adsorption isotherms of nitrogen in NaX and NaCaA are found to be comparable with each other. ‘vl s One has to conclude, therefore, that the translational mobility of nitrogen in the considered zeolites is dominated by the energy of adsorbent-adsorbate interaction rather than by steric reasons. In the latter case the diffusivities in zeolite NaCaA should have been expected to be much smaller than in zeolite NaX. The diffusivities tend to increase with increasing concentration. Such a behavior is well known for zeolitic adsorbate-adsorbent systems with specific interaction.5,‘4 Assuming that the first molecules are attached to the most intense adsorption sites, any enhancement of concentration should enhance the overall mobility within the system. This tendency would be compatible with ZLC measurements of nitrogen in NaCaA and NaX’” where in the limit of negligibly small sorbate concentrations the diffusivities are found to be by one to two orders of magnitude smaller than observed in this study at medium concentrations. It cannot be excluded, however, that this difference might also be an expression of the fact that for a number of systems the diffusivities observed in equilibrium measurements such as p.f.g. n.m.r. significantly exceed the diffusivities derived from nonequilibrium studies.2-4

CONCLUSION For the first time, “N p.f.g. n.m.r. has been applied measure the intracrystalline diffusion of nitrogen

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to in

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lntracrystalline

diffusion

of nitrogen:

N.-K. Blr

et al.

large crystals of NaCaA and NaX zeolite. For temperatures between 135 and 190 K the observed molecular displacements are exclusively determined by intracrystalline diffusion, whereas at higher temperatures, displacements in both the intracrystalline and intercrystalline spaces become significant. The intracrystalline nitrogen diffusivities in zeolite NaCaA and NaX are found to be close to each other, exhibiting a slight increase with increasing concentration. The activation energies of self-diffusion (10.8 + 1.5 kJ/mol for NaX and 8.5 f 1.5 kJ/mol for NaCaA) agree with each other within the limits of accuracy of our measurement.

3

4

5 6 7 8

ACKNOWLEDGMENTS

9

Financial support by Deutsche Forschungsgemeinschaft (Sonderforschungsbereich ‘294) and by Max Buchner-Forschungsstiftung is gratefully acknowledged.

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REFERENCES 1 2

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Callaghan, P.T. Principles of Nuclear Magnetic Resonance Microscopy, Clarendon, Oxford, UK, 1991 KLrger, J. and Ruthven, D.M. Diffusion in Zeolites and Other Microporous So/ids, Wiley, New York, 1992

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Rees, L.V.C., in Weitkamp, J., Klrger, J.H.G., Pfeifer, H. and Helderich, W. (Eds.). Zeolites and Related Microporous Materials: State of the Art 7994, Proceedings of the 70th International Zeolire Conference Part B, Garmisch-Partenkirchen, July 7994, Elsevier, Amsterdam, 1995, p. 1133 Ruthven, D.M. in Zeolites: A Refined Tool for Designing Catalyfic Sites, Proceedings of the International Zeolite Symposium, Quebec, Oct. 7995, (Eds. L. Bonneviot and S. Kaliaguine) Elsevier, Amsterdam 1995, p. 223 KIrger, J. and Pfeifer, H. Zeolites 1987, 7, 90 Klrger, J., Pfeifer, H., Rudtsch, S. and Heink, W. J. fluorine Chem. 1988,39,349 Klrger, J., Pfeifer, H. and Stallmach, F., Feoktistova, N.N. and Zhdanov, S.P. Zeolites 1993, 13, 50 Michel, D., Germanus,A. and Pfeifer, H. J. Chem. Sot. Faraday Trans. 7 1982, 78.237 McDaniel, P.L., Coe, C.G., KBlrger, J. and Moyer, J.D. J. Phys. Chem. 1996,100, 16263 Klrger, J., Bsr, N.-K., Heink, W., Pfeifer, H. and Seiffert, G. Z. Naturforsch. A. 1995,50, 186 Zhdanov, S.P., Khvoshchev, S.S. and Feoktistova, N.N. Synthetic Zeolites, Gordon and Bread, New York, 1990 Btir, N.-K., Ktirger, J., Krause, C., Schmitz, W. and Seiffert, G. J. Magn. Reson. A 1995,113,278 Breck, D.W. Zeolite Molecular Sieves, Wiley, New York, 1974 Hufton, J.R., Ruthven, D.M. and Danner, R.P. Microp. Mat. 1995,5,39 Ruthven, D.M. and Xu, Z. Chem. fng. Sci. 1993, 18.3307